In recent years, according to the increase in carbon-dioxide emissions due to the use of fossil fuel, environmental problems, such as global warming, and energy problems, such as fossil fuel depletion, have attracted public attention. For this reason, the demand for clean energy has been further increasing year by year. As one of the solutions to the problems, there is listed a space solar power generation system (SSPS: Space Solar Power System) project. In the SSPS project, as shown in FIG. 9, an artificial satellite, on which a very large solar battery panel is mounted, is launched at the equator, and the electric power generated from sunlight is converted into microwaves by a power transmission module provided in the solar battery panel. The microwaves 100 are transmitted from a microwave power transmission section 101 to a ground power receiving base station 102 installed on the ground. The transmitted microwaves are utilized on the ground by being again converted into electric power.
Thereby, clean energy can be stably supplied without being subjected to the disadvantages of the solar power generation, that is, without being affected by weather conditions and time zone. As technical problems to realize this project, problems relating to large power transmission, microwave beam control, reduction of operation cost, and the like, are listed. As one of various techniques for solving the problems, a technique for improving the RF-DC conversion efficiency in the ground power receiving base station 102 is listed (see, for example, Patent Literature 1).
In the above-described wireless power transmission system in which electric power is transmitted by being converted into electromagnetic waves, such as microwaves, and in which the electromagnetic waves are received and then electric power is extracted from the received electromagnetic waves, it is generally known that, in order to improve the RF-DC conversion efficiency in a power receiving device, a resistor, which has a resistance value maximizing the RF-DC conversion efficiency with respect to a design value of input power, is connected to a subsequent stage of a rectenna (RF-DC conversion element for converting electromagnetic waves into DC power).